The present invention relates to pneumatic conveying systems and, more particularly, to a vacuum modulating air control apparatus which controls aspects of operation of a vacuum pneumatic conveying system.
Pneumatic conveying systems are frequently used for conveying bulk particulate materials for processing such materials or for feeding such materials into processes which use the materials. So-called dense phase pneumatic conveying systems operate below the minimum suspension velocity for a given material and are characterized by the product moving along the bottom of a pipe in “slugs” or a moving bed. The product is conveyed at high differential pressures below its saltation velocity, that is, the gas flow velocity below which the material begins to settle out of the gas flow. Conveying velocities are typically less than 2600 FPM (feet per minute), and material-to-air ratios of up to 100 to 1 are possible. Interaction of the particles in a dense phase system is greater than in a dilute phase conveying systems. Common applications for dense phase conveying systems include products with heavy bulk-densities, abrasive products, friable products, blended products, and, in some instances, products that do not require continuous delivery to their destinations.
Vacuum dense phase pneumatic conveying systems employ high vacuum pumps and special feeding mechanisms to form moving slugs of material which are conveyed at sub-atmospheric pressures. The result is minimized pipe friction and maximum protection of the product for multiple applications. Products commonly conveyed in vacuum dense phase systems include: friable products, blended products, and abrasive materials. These products may include sugar, pet food products, carbon black prills, plastic pellets, detergents, and the like. Depending on the product to be conveyed, variations in the configuration of the conveying system may be necessary with respect to such features as size and material characteristics of conveying conduits, structural support of the conduits, vacuum levels, conveying velocities, and the like.
There are two conventionally used methods for feeding products to be conveyed into a vacuum conveying system. In one method, an impulse valve alternates between drawing material and air into the conduit on a timed basis to create the desired material-to-air ratio. In a second method, a series of static bleed valves is used to balance the ratio of material and air.